1. Field of the Invention
This invention relates generally to synchronizing signal detecting circuits, and more particularly is directed to a synchronizing signal detecting circuit for digital data reproduced from a magnetic disc.
2. Description of the Prior Art
A standardized small-sized floppy disc having a 2-inch diameter, and which was developed for use in recording video signals in association with an electronic still camera, has also been proposed as a recording medium for digital data.
Such standardized video floppy disc 1 is shown in FIG. 1 to include a magnetic disc. This magnetic disc 2 is 47 mm in diameter, 40 .mu.m in thickness and is provided at its center with a core 3 engageable with a spindle of a disc drive apparatus (not shown). The center core 3 is provided with a magnetic insert 4 which is detectable to present a reference signal indicating the angular position of the magnetic disc 2 when it is rotated.
A receptacle or jacket 5, which is 60.times.54.times.3.6 mm in size, rotatably contains the magnetic disc 2. The jacket 5 includes a central opening 5A to expose therethrough the center core 3 and the magnetic insert 4 to the outside. The, jacket 5 is further provided with an access opening on window 5B, through which a magnetic head (not shown) can contact the magnetic disc 2 during recording and/or reproducing. When the video floppy disc 1 is not in use, access opening or window 5B is closed by a slidable dust-proof shutter 6. A nail member or tab 7 is provided on the jacket 5 for avoiding inadvertent or erroneous recording. This nail member 7 is removed from the jacket 5 when recording is to be inhibited.
In the recording mode, 50 concentric magnetic tracks can be formed on the magnetic disc 2 in which case, the outermost track is represented as the 1st track and the innermost track is represented as the 50th track. The width of each track is 60 .mu.m and the width of each guard band between the tracks is 40 .mu.m.
When taking a picture by means of the electronic still camera, the magnetic disc 2 is rotated at 3600 rpm (field frequency) and a video signal of one field is recorded in one circular track as a still picture. The color video signal to be thus recorded has the frequency distribution shown in FIG. 2. More particularly, a luminance signal Sy is shown to be frequency-modulated to an FM signal Sf, with the sync tip level of the FM signal Sf being 6 MHz and the white peak level thereof being 7.5 MHz. Further, a line sequential color signal Sc is formed of a frequency-modulated signal Sr having a carrier with a central frequency of 1.2 MHz modulated by a color difference signal R-Y and of a frequency-modulated signal Sb having a carrier with central frequency of 1.3 MHz modulated by a color difference signal B-Y. A composite signal Sa resulting from adding the frequency-modulated color signal Sc and the frequency-modulated luminance signal Sf is recorded on the magnetic disc 2.
The video floppy disc 1 shown in FIG. 1 has a proper size and characteristics to act as a recording medium for the color video signal Sa of FIG. 2. It has also been proposed that the video floppy disc 1 may be used as a recording medium for recording digital data, and FIGS. 3A to 3D illustrate previously proposed physical formats to be used when, digital data are recorded on and/or reproduced from the video floppy disc 1.
In FIGS. 3A and 3B, reference TRCK designates one of the tracks formed on the magnetic disc 2 in the digital data recording format. This track TRCK comprises a gap area or interval GAP2 of 2.degree. angular extent, an index area or interval INDX of 2.degree. angular extent following GAP2 in the longitudinal direction of the track TRCK and four equally-divided intervals of 89.degree. based on the position of the magnetic insert 4 as a reference. Each of the four 89.degree. intervals is referred to as a sector SECT. The sector SECT immediately after the magnetic insert 4 is referred to as zero-th sector (#0) and other succeeding sectors SECT are sequentially referred to as the first (#1), second (#2) and third (#3) sectors, respectively. When data are interchanged between the video floppy disc 1 and a host computer (not shown), that data are interchanged with one sector SECT as the unit. Further, the index interval INDX corresponds to about three of the frame intervals of data indicated at FRAM and which will be described later. In the example being described, a signal "1000", indicative of Tmax (maximum length between transitions) of a digital signal, is repeatedly recorded all over the index interval INDX.
As shown in FIG. 3C, the interval of 2.degree. from the start end of each sector SECT is provided as a gap interval GAP1 that is used as a margin portion during read and write operations. The remaining portion of each sector SECT is divided equally into 131 intervals in each of which 44 channel bytes are recorded and/or reproduced. Each such channel byte is the unit of a signal formed by an eight-to-ten conversion and corresponds to one byte of source data, while it corresponds to 10 bits in the eight-to-ten conversion. The first two of the 131 equal intervals are provided as preamble sections PRAM. In the preamble sections PRAM, there is repeatedly provided a signal of "0101010101" which corresponds to, for example, 00H (H is a hexadecimal notation) of a source data and which is used for locking-in operation of a PLL (phase locked loop) circuit in the playback mode.
The 128 equal intervals following the preamble sections PRAM are referred to as frame intervals FRAM in which digital data are recorded and/or reproduced. The last one of the 131 equal intervals is used as a post-amble section PSAM which is equivalent to the preamble section PRAM.
As shown in FIG. 3D, one frame interval FRAM sequentially comprises, from its beginning, a synchronizing signal SYNC ("0100010001" or "1100010001") of one channel byte, a frame address signal FADR of one channel byte, a non-defined signal NRSV of one channel byte, a check signal FPTY of one channel byte, data DATA of 32 channel bytes and first and second redundant data PRT1 and PRT2 each of which is formed of 4 channel bytes, in the order stated. In such case, the check signal FPTY acts as parity data for the frame address signal FADR and the non-defined signal NRSV. While the data DATA are original data which are accessed by the host computer, this data DATA are interleaved within the digital data of one sector SECT. The redundant data PRT1 and PRT2 are parity data that are generated by coding digital data of one sector (32 bytes.times.128 frames) by the minimum distance 5 according to the Reed Solomon coding method.
Accordingly, the capacities for digital data in one sector SECT, one track TRCK and one video floppy disc 1 are as follows:
One sector : 4096 bytes (=16 bytes .times.2.times.128 frames) PA0 One track : 16K bytes (=4096 bytes .times.4 sectors) PA0 One floppy disc : 800K bytes (=16K bytes .times.50 tracks) PA0 One frame : =(4+32+4+4) bytes .times.8 source bits =352 source bits PA0 One frame : 440 channel bits PA0 One sector (except gap interval GAP1) : 57640 channel bits PA0 14.31M bits/second (.congruent.58965 bits.times.4 blocks.times.field frequency.times.360.degree./356.degree.)
When digital data are accessed on the video floppy disc 1, such accessing is carried out with one sector SECT as the unit so that the accessing of digital data on the video floppy disc 1 is effected on the basis of a unit of 4K bytes.
Further, the bit numbers of one frame FRAM and one sector SECT are as follows:
One sector (except gap interval GAP1) =352 bits .times. (128+3 frames)=46112 source bits
In practice, when the digital data are recorded on and/or reproduced from video floppy disc 1, the value DSV (digital sum value) must be made small, the value of Tmin/Tmax must be made small and the value of Tw (window margin) must be made large. In order to satisfy the foregoing requirements, all the digital signals are first subjected to the above mentioned eight-to-ten conversion using Tmax=4T and then recorded on the video floppy disc 1. Upon reproducing the digital signals, they are subjected to the reverse conversion (eight-to-ten conversion) and then subjected to the original signal processing.
Accordingly, for the data densities given above, the practical bit number on the video floppy disc 1 is multiplied by 10/8 and amounts to the following:
Thus, the whole interval of one sector SECT is equivalent to 58965 channel bits (.congruent.57640 channel bits.times.89.degree./87.degree.). In practice, since the length of each interval is assigned from this channel bit number, as described above, the total length of each sector SECT comprised of frame intervals FRAM is slightly shorter than 87.degree..
Accordingly, the bit rate used when digital data (signal converted according to the eight-to-ten conversion) are accessed on the video floppy disc 1 is
and one bit is equivalent to 69.9 nano-seconds (.congruent.1/14.31 M bits).
A video signal and digital data can be recorded on the same video floppy disc 1 if they are recorded thereon with the track as a unit, that is, each track is recorded either with a video signal or digital data.
With the above described format, digital data of 800K bytes can be recorded on and/or reproduced from one side of the 2-inch size video floppy disc 1. This capacity is more than twice the normal capacity of the known 5-inch size floppy disc. Therefore, this 2-inch video floppy disc has a large capacity even though it is small in size.
Further, since the rotational speed of the magnetic disc 2 is the same as that used in the case of the video signal, the video signal and the digital data can be recorded and reproduced on the same disc. In that case, the frequency spectra of both types of signals recorded and/or reproduced from the magnetic disc 2 become similar so that they can be recorded and reproduced under similar suitable conditions, such as, the electromagnetic transducer characteristics, head contact with the tape and the like. Furthermore, when the two types signals are recorded and/or reproduced in a mixed state, the rotational speed of the magnetic disc 2 does not have to be changed so that it is not necessary to consider the time necessary for switching the servo circuit. Hence, the two types of signals can be immediately used separately. In addition, the facts that only one rotational speed is used and that it is sufficient that the electromagnetic transducer system and the like have only one characteristic or function, are also advantageous from the standpoint of cost.
Thus, the described video floppy disc 1 has novel effects as a medium for recording and reproducing a video signal or for storing digital data, or as a medium for recording and reproducing a video signal and digital data on the same disc.
In the above mentioned example, when the source data are derived from the video floppy disc 1, taking the position of the synchronizing signal SYNC as a reference, the channel data are divided into channel data of 10 bits each and the divided channel data are decoded to the original source data according to the ten-to-eight conversion. Accordingly, if the synchronizing signal SYNC is detected erroneously at an improper position, the succeeding channel data are divided at the wrong positions, that is, a bit slip occurs so that an error occurs in the source data until the correct synchronizing signal is again detected. If the distance of the synchronizing signal SYNC relative to the source data is large, the probability that such error will occur is small. If, on the other hand, the transmission band is compressed as mentioned above, such distance is as short as one bit so that there is a large probability that error will occur. If the described error occurs frequently, the resulting errors can no longer be corrected by the first and second parity data PRT1 and PRT2.
Further, in the above mentioned eight-to-ten conversion, 84H assumes "1010001001" and 80H assumes "1010010101" in, for example, the source data and it is assumed that they are successive. If the last bit "1" of 84H is mistaken as "0", a bit slip of "X100010001" occurs and this bit slip coincides with the synchronizing pattern so that a synchronizing error occurs.
Here, the probability Pse of the synchronizing error is presented as Pse=Pbe*Pr14*Pr13 where Pbe is the bare bit error rate, PR14 is the probability of 4-bit run length error in the eight-to-ten conversion and PR13 is the probability of 3-bit run length error in the eight-to-ten conversion. By way of example, if Pbe=10.sup.-4, PR14=0.062 and Pr13=0.213, and the probability Pse becomes: EQU Pse .apprxeq.1.3.times.10.sup.-6
Accordingly, if the error correction ability of the above mentioned respective redundant bits provides a system in which 10.sup.-12 can be handled by Pbe=10.sup.-4, the Pse becomes considerably lower than the above value and hence this is disadvantageous.
In other words, if the probability that the synchronizing error will occur is considerably higher than the data error correction ability, the ability of the system to correct errors is overwhelmed.